www.nature.com/scientificreports OPEN The role of the environment in the evolution of nest shape in Australian passerines Received: 14 August 2018 Iliana Medina Accepted: 21 March 2019 Avian nests present great variation in structure but, after excluding cavity nesters, probably the Published: xx xx xxxx most obvious diference is that between open and domed nests. Some species lay their eggs in open structures, exposed to environmental variables, while other species build domed, enclosed nests with a roof, which are suggested to protect eggs and nestlings from weather conditions, high radiation levels, and predation. To date it is unclear which variables drove the evolution of diferent nest types. In this study, environmental and nest type information was extracted for continental Australian passerines, showing that species with open and closed nests are distributed in similar climates. However, species with open nests have larger ranges and are distributed in a wider variety of climatic conditions, suggesting open nests could be an evolutionary key innovation. This analysis was complemented with a detailed study of the evolution of particular nest traits in the largest Australasian avian radiation (Meliphagoidea), confrming that adult body size – but not environment – is an important factor in nest architecture, and larger species tend to build nests that are shallow and supported from underneath. Nest structure is a multidimensional trait that has probably evolved to match the phenotype of the nest owner, but that could also constrain or facilitate establishment in diferent environments. Bird nests have fascinated biologists for centuries due to the wide diversity of nest types, and many explanations have been proposed to explain such variation. Aristotle recognized that birds that lay their eggs on the ground were usually poor fyers, and Wallace and Darwin proposed that nesting in holes had evolved in colourful birds to decrease conspicuousness while incubating1,2. Although to date there is very detailed information about nest- ing habits and nest morphology in thousands of species3,4, we still know little about how these structures have evolved5. Nests are an ideal trait to study in an evolutionary context, because they are considered to be highly conserved within species and within families6,7. Although nests are taxonomically conserved, this does not mean that they are not under strong selective pressures. Besides phylogenetic inertia, biotic and environmental factors may have also driven the evolution of these structures5,8. Predation pressures have been proposed as one important biotic factor behind the evolution of domed nests4,9,10. For example, in Old World babblers (Timaliidae) it has been suggested that domed nests evolved as a way to protect eggs and nestlings from predation on the ground8. Domed nests are structures that are enclosed and usually have a small entrance, unlike cup nests where the top of the nest lacks a roof and thus eggs and nestlings are completely exposed if the incubating parent is not present9,11. Te size of the owner of the nest is another important biotic factor in determining nest architecture12,13. It is known that nest mass increases with species mass14, but larger species may encounter restrictions in certain nest morphologies (e.g. domed) due to the amount of material needed13,15. Another variable that can potentially be associated with nest architecture is the environment4,16. Domed nests may have evolved in colder environments as an adaptation to increase nest temperatures thus decreasing incu- bation time and protecting eggs and nestlings from the cold13,17. Conversely, dome nests may have also evolved to decrease egg damage caused by radiation and heat stress in hot and arid places12,18,19. Moreover, nest type may not only respond to environmental pressures in the habitat, but the type of nest a bird builds can also impact their ftness, distribution and dispersal capacities. For instance, species that nest in cavities are highly limited by nesting sites, and the distribution of these sites can constrain their dispersal20,21. If building domed nests ofers protection from radiation, temperature, or predation, then it is possible that species with domed nests may be able to succeed in a wider variety of environments and may be able to disperse across larger ranges. Alternatively, domed nests School of BioSciences, University of Melbourne, Parkville, 3010, VIC, Australia. Correspondence and requests for materials should be addressed to I.M. (email: [email protected]) SCIENTIFIC REPORTS | (2019) 9:5560 | https://doi.org/10.1038/s41598-019-41948-x 1 www.nature.com/scientificreports/ www.nature.com/scientificreports might be so specialized (i.e. adapted to a relatively narrow set of environmental conditions) that this could result in restriction of the dispersal and distribution of the species. Price and Grifth7 recently discovered that in the history of perching birds (Passeriformes), there were multi- ple transitions from having an enclosed nest to having an open nest. However, it is still unclear why this transition occurred and the evolutionary implications of this change in nest type. Te frst and main aim in this study is to build on Price and Grifth’s study and use their information on broad nest type (open or domed) in all Australian continental passerines to test whether environmental variables are correlated with the evolution of nest type in this continent. Cavity nesters were excluded because there are few species in Australia, and because the building process of these nests and the habitat requirements are diferent from those of domed nests. Australia is an ideal place to test the link between nest types and environment because it has wide geographic variation in climate, including warm arid zones and relatively low temperatures in winter in some places22,23. Although Australian avifauna is exposed to diferent climates, there is a high likelihood that all share relatively similar biotic condi- tions (e.g. are exposed to similar predators, brood parasites, and fora), at least compared to species outside the continent. Tis provides a more controlled environment for hypotheses testing12,23. Additionally, phylogenetic analyses suggest that passerines originated and radiated in Australasia, and Australia presents a large number of the passerine families of the world24,25, which makes the present results relevant in a wider evolutionary context. Studies on nest architecture mostly focus on the division between domed and open nests, because this general information is available for a great number of species, but in reality, there is variation within these broad cate- gories7,8,26,27, and studying this variation can lead to important insights in the evolution of nests. For instance, some species hang or suspend their nests on branch rims on forks of trees, while other species support their nests from underneath27. Some species also have nests that are deep and similar to a pouch, while other species build saucer-shaped nests or platforms. A recent study found that two traits, the broad shape of the nest and the location of the nest, have disparate evolutionary trajectories27. Some of these traits may have an adaptive value. In Europe it was shown that a larger species of tit (Parus major) tends to build shallower nests than a smaller species (Cyanistes caeruleus), but only inside smaller and warmer chambers28. Considering nests as a multidimensional trait under selective pressures may be needed to fully understand their evolution6,29. In Australasia, honeyeaters and allies (superfamily Meliphagoidea) ofer a great opportunity to study the evolution of particular nest traits in more detail, because they are a speciose monophyletic group with high variation in nest types and have detailed information on nest morphology available. Some species in this superfamily build open nests, other domed nests and in both cases, these can be either attached to a surface (supported by branch or bush) or suspended from the upper part. For the second aim, this Australasian superfamily was used to understand in greater depth the evolution of nest architecture in birds, by scoring nest type as a composite of eight traits. Environmental and morphological variables of the species were used to explore which are the drivers or particular nest features and to reconstruct the ancestral state of diferent nest traits that describe overall nest architecture. In this study I build on the previous work of Price and Grifth7 and collate their published nest type informa- tion for Australian passerines (~70%) and use it in a series of comparative analyses to test: (1) whether broad nest shape (open or closed) can be predicted by environmental variables (temperature and radiation) in Australian passerines and (2) whether species with diferent nest shape present diferences in range size and niche breadth, quantifed as climatic variation across the species range. Ten, I collect information on eight diferent nest traits and climatic information in a subsample of honeyeaters and allies to explore how diferent traits of nest shape evolve in a phylogenetic context, and whether environment can predict specifc nest traits. A principal coordinate analysis was used to visualize the nest shape space used by each species and to investigate how diferent traits are related to each other. Tis information was further used to understand how environment and other traits such as mass and nest height can afect each diferent